Re: [PATCH v3 4/7] memcg: remove memcg from the reclaim iterators

From: Johannes Weiner
Date: Tue Feb 12 2013 - 10:10:31 EST

On Tue, Feb 12, 2013 at 10:54:19AM +0100, Michal Hocko wrote:
> On Mon 11-02-13 17:39:43, Johannes Weiner wrote:
> > On Mon, Feb 11, 2013 at 10:27:56PM +0100, Michal Hocko wrote:
> > > On Mon 11-02-13 14:58:24, Johannes Weiner wrote:
> > > > That way, if the dead count gives the go-ahead, you KNOW that the
> > > > position cache is valid, because it has been updated first.
> > >
> > > OK, you are right. We can live without css_tryget because dead_count is
> > > either OK which means that css would be alive at least this rcu period
> > > (and RCU walk would be safe as well) or it is incremented which means
> > > that we have started css_offline already and then css is dead already.
> > > So css_tryget can be dropped.
> >
> > Not quite :)
> >
> > The dead_count check is for completed destructions,
> Not quite :P. dead_count is incremented in css_offline callback which is
> called before the cgroup core releases its last reference and unlinks
> the group from the siblinks. css_tryget would already fail at this stage
> because CSS_DEACT_BIAS is in place at that time but this doesn't break
> RCU walk. So I think we are safe even without css_get.

But you drop the RCU lock before you return.

dead_count IS incremented for every destruction, but it's not reliable
for concurrent ones, is what I meant. Again, if there is a dead_count
mismatch, your pointer might be dangling, easy case. However, even if
there is no mismatch, you could still race with a destruction that has
marked the object dead, and then frees it once you drop the RCU lock,
so you need try_get() to check if the object is dead, or you could
return a pointer to freed or soon to be freed memory.

* If the dead_count mismatches, a destruction has happened or is
* happening concurrently. If the dead_count matches, a destruction
* might still happen concurrently, but since we checked under RCU,
* that destruction won't free the object until we release the RCU
* reader lock. Thus, the dead_count check verifies the pointer is
* still valid, css_tryget() verifies the cgroup pointed to is alive.
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